SUMMARY OF WORK Software development on the full 3D model is proceding. The 2D grid model has been revised to include detailed balancing of whole-cell calcium balance. This model has been entirely recoded in compiled Java and spun out to run concurrently on all 48 processors of our local cluster with >70% efficiency. This has made it possible to explore parameters and identify several different new regimes of propagation relevant to the pacemaking function of SR calcium release in sin0-atrial node cells. Extensive simulations have been performed as a function of critical parameters regulating heart rate. The model successfully expalins the various regimes of localized and global calcium release and the transition among these regimes as a function of release current. These results are consistent with a new model of how heart rate is regulated by the autonomic nervous system. A publication is presently being reviewed by journal.